The present invention generally concerns a motor protection relay using thermal models, specifically including a thermal model for the start condition of a motor protection relay and a thermal model for the run condition of the relay. Such a relay is described in U.S. Pat. No. 5,436,784, which is owned by the assignee of the present invention, the contents of which are hereby incorporated by reference.
Briefly, an induction motor operates in either a high current “start” condition or a relatively low current “run” condition. As defined, in the motor's start condition, the current in the rotor is greater than 2.5 times the full load current, while in the motor's run condition, the current in the rotor is less than 2.5 times the full load current.
The thermal model for the motor's start condition, during which time current is flowing through the rotor portion of the motor without the rotor actually moving, is different than the thermal model from the run condition, when the motor is running at its operating speed. The thermal models can be represented by electrical analog circuits, shown for the start condition in FIG. 1A and the run condition in FIG. 1B. In circuit 10 of FIG. 1A, the heating effect in the motor is represented by three elements, a generator 12, the thermal capacity of the rotor 14 and a cooling effect member 16, i.e. the ability of the motor to give off heat. In the start condition, however, the ability of the motor to give off heat is zero.
FIG. 1A shows generator 12 to be represented by a voltage, the thermal capacity of the rotor 14 by a capacitance and the cooling effect 16 (if there is one) by a resistance. The total heating/cooling effect from elements 12, 14, and 16 (although there is in fact no cooling effect 16 for the start condition) is then applied to a comparator 17, which compares the signal information represented by the outputs of the heat generator and the capacitor, with a threshold value. The output of the comparator is a trip signal if the threshold is exceeded.
An existing thermal model for the run condition is shown in FIG. 1B, which shows a heat generator 18, a capacitor 20 which represents the thermal capacity of the system and a cooling effect element 22. In both cases, R1 is the locked rotor electrical resistance, R0 is the running rotor electrical resistance, IL is equal to the locked rotor current, Ta is the locked rotor time with the motor initially at an ambient temperature, and T0 is the locked rotor time with the motor initially at an operating temperature. Typically, R1/R0 is equal to 3, while Ta/To typically will be 1.2 (the service factor).
Typically, the transition from the higher trip threshold of the start condition to the lower trip threshold of the run condition, representing a “cooling” of the trip threshold because of the overall cooler temperatures in the run condition, is based on an exponential decline using the following formula during transition:
      U    th    =                    T        R            ⁡              (        run        )              +                  (                                            T              R                        ⁢            start                    -                                    T              R                        ⁢            run                          )            ·              ⅇ                  -                      t            RC                              However, when there is a transition between the run condition to the start condition, the threshold change is made substantially instantaneously so that the trip threshold remains ahead of the steeply rising current present in the start (stall) condition of the motor when the rotor is not turning.
As explained in more detail below, an immediate change to the trip threshold for the start condition is sometimes disadvantageous, since the cause of such a transition might be a temporary current spike or a very short-term current rise, instead of the motor actually going into a true start (or stall) condition. Since the trip threshold for the start condition is considerably higher than the run condition, the use of the start condition threshold when the motor is actually in the run condition is not particularly desirable from a protection standpoint. Once the start condition trip threshold is initiated, a relatively long period of time is required while the threshold decreases over time, by virtue of the exponential transition time set forth in the '784 patent.
Accordingly, the trip threshold will be too high for the run condition for a period of time, thus providing possibly insufficient protection for the motor during that time, because the motor could be allowed to heat higher without tripping during the run condition than the true, established trip threshold for the run condition would ordinarily allow. Hence, it would be desirable to provide another, more reliable basis for transitioning to the start condition trip threshold.